Fair Value Measurements - Investments by Contractual Maturity Date (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019
Amortized Cost, Available-for-sale securities
Amortized Cost	$ 1,470,063	$ 1,458,071
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains	29,472	30,461
Gross unrealized loss	(3,949)	(2,790)
Fair Value, Available-for-sale securities
Fair value	$ 1,495,586	$ 1,485,742
Yield, Available-for-sale securities
Yield	3.11%	3.27%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, amortized cost	$ 1,360,694	$ 1,443,480
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	20,024	19,952
Held-to-maturity securities, gross unrealized losses	(298)	(337)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 1,380,420	$ 1,463,095
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	3.14%	3.15%
Investments	$ 2,830,757	$ 2,901,551
Gross unrealized gains on investments	49,496	50,413
Gross unrealized losses on investments	(4,247)	(3,127)
Total financial assets	$ 2,876,006	$ 2,948,837
Yield on investments	3.12%	3.21%
U.S. government and agency securities
Amortized Cost, Available-for-sale securities
Amortized cost, 5 to 10 years	$ 63,512	$ 65,287
Amortized cost, over 10 years	272,314	207,067
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 5 to 10 years	2	39
Gross unrealized gains, over 10 years	14	1
Gross unrealized losses, 5 to 10 years	(979)	(629)
Gross unrealized losses, over 10 years	(1,349)	(987)
Fair Value, Available-for-sale securities
Fair value, 5 to 10 years	62,535	64,697
Fair value, over 10 years	$ 270,979	$ 206,081
Yield, Available-for-sale securities
Yield, 5 to 10 years	2.27%	2.43%
Yield, over 10 years	2.82%	3.02%
Corporate debt securities
Amortized Cost, Available-for-sale securities
Amortized cost, within 1 year	$ 43,938	$ 43,903
Amortized cost, 1 to 5 years	70,000	70,000
Amortized cost, 5 to 10 years	93,008	92,931
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, within 1 year	307	411
Gross unrealized gains, 1 to 5 years	977	689
Gross unrealized gains, 5 to 10 years	2,419	1,879
Gross unrealized losses, Within 1 year	0	0
Gross unrealized losses, 1 to 5 years	0	(50)
Gross unrealized losses, 5 to 10 years	0	0
Fair Value, Available-for-sale securities
Fair value, within 1 year	44,245	44,314
Fair value, 1 to 5 years	70,977	70,639
Fair value, 5 to 10 years	$ 95,427	$ 94,810
Yield, Available-for-sale securities
Yield, within 1 year	3.41%	3.65%
Yield, 1 to 5 years	3.01%	3.29%
Yield, 5 to 10 years	2.95%	3.27%
Municipal bonds due
Amortized Cost, Available-for-sale securities
Amortized cost, 1 to 5 years	$ 1,437	$ 1,430
Amortized cost, over 10 years	20,298	20,303
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, 1 to 5 years	23	14
Gross unrealized gains, over 10 years	732	895
Gross unrealized losses, 1 to 5 years	0	0
Gross unrealized losses, over 10 years	0	0
Fair Value, Available-for-sale securities
Fair value, 1 to 5 years	1,460	1,444
Fair value, over 10 years	$ 21,030	$ 21,198
Yield, Available-for-sale securities
Yield, 1 to 5 years	1.95%	1.94%
Yield, over 10 years	6.45%	6.45%
Agency pass-through certificates
Amortized Cost, Available-for-sale securities
Amortized cost, without single maturity date	$ 905,556	$ 957,150
Gross Unrealized Gains / Losses, Available-for-sale securities
Gross unrealized gains, without single maturity date	24,998	26,533
Gross unrealized losses, without single maturity date	(1,621)	(1,124)
Fair Value, Available-for-sale securities
Fair value, without single maturity date	$ 928,933	$ 982,559
Yield, Available-for-sale securities
Yield, without single maturity date	3.19%	3.29%
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, amortized cost	$ 1,348,594	$ 1,428,480
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	20,018	19,945
Held-to-maturity securities, gross unrealized losses	(298)	(337)
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 1,368,314	$ 1,448,088
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	3.15%	3.15%
Commercial MBS
Amortized Cost, Held-to-maturity securities
Held-to-maturity securities, amortized cost	$ 12,100	$ 15,000
Gross Unrealized Gains / Losses, Held-to-maturity securities
Held-to-maturity securities, gross unrealized gains	6	7
Held-to-maturity securities, gross unrealized losses	0	0
Fair Value, Held-to-maturity securities
Held-to-maturity securities, fair value	$ 12,106	$ 15,007
Yield, Held-to-maturity securities
Held-to-maturity securities, yield	2.61%	2.89%

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).